Normally gaseous materials are cooled for a variety of purposes. Cryogenic liquefaction of normally gaseous materials is utilized, for example, in separation of mixtures, purification of the component gases, storage and transportation of the normally gaseous material in an economic and convenient form, and other uses. Most such liquefaction processes have many operations in common, whatever the particular gases to be liquefied, and consequently have many of the same operating problems. One common problem is the compression of refrigerants and/or components of the normally gaseous material. Accordingly, the present invention will be described with specific reference to processing natural gas, but is applicable to processing of other gases.
It is common practice in the art of processing natural gas to subject the natural gas to cryogenic treatment to separate hydrocarbons having a molecular weight higher than methane from the natural gas. Thereby, pipeline gases predominating in methane, and a gas predominating in higher molecular weight components for other uses are produced. It is also common practice to cryogenically treat natural gas to liquefy the same for transportation and storage.
Processes for the liquefaction of natural gas are principally of two main types. The most efficient and effective type is an optimized cascade operation, and this optimized type in combination with expansion type cooling. The cascade process provides a series of refrigerants selected so as to provide only small temperature differences between the refrigeration system and the natural gas being cooled. In this manner it closely matches the cooling characteristics of the natural gas feed. By using a sequence of refrigerants the natural gas is cooled from ambient temperature as received from wells or pipelines down to about -259.degree. F., which is typical of LNG. The second type process, which is less efficient, uses multi component refrigerant cycles to approximate the cascade process.
In the cascade-type of cryogenic production of LNG, the natural gas is first subjected to preliminary treatment to remove acid gases and moisture. Natural gas at an elevated pressure, either as produced from the wells or after compression and at approximately atmospheric temperature, is cooled in a sequence of multistage refrigeration cycles by indirect heat exchange with two or more refrigerants. For example, the natural gas is sequentially passed through multistages of a first refrigerant cycle, which employs a relatively high boiling refrigerant, such as propane. It is then passed through multi stages of a second cycle in heat exchange with a refrigerant having a lower boiling point, for example ethane or ethylene, and finally through a third cycle in heat exchange with a refrigerant having a still lower boiling point, for example methane.
In each stage of the high and intermediate cooling stages of a three-stage refrigerant compressor system, the natural gas is cooled by compressing the refrigerant to a pressure at which it can be liquefied by cooling. The liquefied refrigerant is then expanded to flash part of the liquid into the shell of a high-stage core-in-shell heat exchanger. This, of course, requires larger than normal shells for the heat exchanger. The feed gas stream passes through the core of the exchanger while the refrigerant is expanded into the shell cooling the refrigerant stream. The gaseous portion passes through the shell vapor space and exits the shell. The liquid phase is collected in the shell. The liquid phase is then circulated to contact the cores by thermosiphon circulation. Approximately 25 to 30% of the thermosiphon circulated fluid evaporates providing the cooling for indirect heat exchange with the feed gas. The heat exchanger shell can also function as separator for separating the flashed gas from the remaining liquid. Remaining liquid in the first chiller is then further expanded to flash a second portion of the liquid into an intermediate stage of the cooling cycle. The remaining liquid from the intermediate stage heat exchanger shell may be further expanded to flash a third portion of the liquid in a low stage of the cooling cycle. Accordingly, a multistage refrigeration compressor system typically includes a very large volume low stage core-in-shell heat exchanger (because of the large low-stage vapor-compression refrigeration service), and relatively small volume high and interstage core-in-shell exchangers because of the reduced vapor-compression refrigeration service required for these stages.
A problem arises in this heat exchanger configuration, however, in that small liquid level upsets in the large volume low stage shells have a very large destabilizing effect on the liquid level required for the much smaller high-stage and intermediate-stage cores.
Accordingly, it is an object of this invention to improve the apparatus and method used for cooling a normally gaseous material.
Another object of this invention is to improve operating efficiency of a multistage compression refrigeration cycle.
It is a more specific object to improve stability of refrigerant liquid levels in plate fin core-in-shell heat exchangers in a multistage compressor system.